1. Field of the Invention
The present invention relates to a reversed-polarity pulse generating circuit for direct current plasma, which generates a reversed-polarity voltage pulse being effective for suppressing or preventing occurrence of abnormal electrical discharge such as arc discharge in a direct current plasma device.
2. Related Art
Conventionally, in a direct current plasma device that utilizes a plasma discharge generated in vacuum, such as a sputtering device, a PVD (Physical Vapor Deposition) device, a CVD (Chemical Vapor Deposition) device, and an etching device, impedance between electrodes in the plasma load may be reduced, or foreign matter such as conductive dust may result in a short circuit between the electrodes. In such a case, the plasma discharge occurring in vacuum in the plasma load may temporarily become an abnormal electrical discharge such as an arc discharge. In particular, if such an abnormal electrical discharge occurs in a sputtering device, the target may melt. In a sputtering device, for example, if the target melts while sputtering a substrate material such as a liquid crystal panel, the substrate material becomes defective. Moreover, for example, if the target melts while sputtering a metal film of a CD or DVD, the metal film becomes defective. Such issues bring about a problem of deteriorating product yield. Therefore, the occurrence of such abnormal electrical discharge must be suppressed as much as possible.
With reference to, for example, Japanese Unexamined Patent Application, Publication No. 2005-347212 (hereinafter referred to as Patent Document 1), a conventional method as a countermeasure against abnormal electrical discharge in a direct current plasma device is to extinguish abnormal electrical discharge in a short time by applying a reversed-polarity voltage pulse to the plasma load when an occurrence of an abnormal electrical discharge has been detected. In addition, as another countermeasure against abnormal electrical discharge, a method is disclosed which prevents occurrence of abnormal electrical discharge by periodically applying reversed-polarity voltage to the plasma load as described above.
Furthermore, with reference to Japanese Unexamined Patent Application, Publication No. 2006-274393 (hereinafter referred to as Patent Document 2), another method as a countermeasure against abnormal electrical discharge is disclosed, which is a technique in which a phenomenon immediately before the occurrence of abnormal electrical discharge is detected as a predictor of the occurrence of abnormal electrical discharge, and reversed-polarity voltage is applied to the plasma load immediately before the occurrence of abnormal electrical discharge as previously described, thereby preventing the occurrence of abnormal electrical discharge.
The methods as countermeasures against abnormal electrical discharge as described above are effective in that abnormal electrical discharge having occurred can be immediately extinguished, and abnormal electrical discharge can be prevented from occurring. In this way, various configurations have already been proposed for implementing methods as countermeasures against abnormal electrical discharge.
Furthermore, for example, with reference to Japanese Unexamined Patent Application, Publication No. H10-298755 (hereinafter referred to as Patent Document 3), a circuit is used, in which an additional winding wire, magnetically coupled to inductance means for stabilizing an electric current flowing through a plasma load, is serially connected with a switching element for reversed-polarity voltage application. In this circuit, when an abnormal electrical discharge occurs in a plasma load, a switching element for reversed-polarity voltage application is made electrically conductive, the inductance means generates a positive voltage pulse with reversed-polarity in the inductance means, the voltage value thereof being higher than the voltage value of the negative voltage generated in the inductance means, and a reversed-polarity voltage pulse is applied to the plasma load by utilizing the positive voltage with the reversed-polarity. As a result, abnormal electrical discharge is promptly extinguished.
In a case of a power supply unit for direct current plasma, when the direct current voltage that is output from the power supply unit for direct current plasma in a plasma discharging state is Vo, the power supply unit is in a substantially unloaded state before plasma discharge (plasma ignition), and outputs direct current high voltage of 1.5 to 1.8 Vo that is necessary for generating plasma discharge. Therefore, not only the invention of Patent Document 3 but also the aforementioned switching elements for reversed-polarity voltage application used in the methods as countermeasures against abnormal electrical discharge must have a breakdown voltage capable of withstanding direct current high voltage of at least 1.5 to 1.8 Vo.
In the invention of Patent Document 3, although a power supply unit for supplying special reversed-polarity voltage is not required, the leakage inductance between the winding wires of the inductance means, and the surge voltage due to the wiring inductance are superimposed on the reset voltage that is necessary for resetting the inductance means. In order to reduce the influence of the surge voltage, a voltage surge absorption circuit must be connected in parallel with the switching element for reversed-polarity voltage application. Such a voltage surge absorption circuit is composed of a resistor and a capacitor with sufficient current-carrying capacity, and increases the power loss. Particularly in a case in which the switching element for reversed-polarity voltage application is periodically switched, there is a problem in that power loss and heat generation of the voltage surge absorption circuit increase. Even if such a voltage surge absorption circuit is provided, a switching element for reversed-polarity voltage application with rated direct current high voltage being approximately 2.5 to 3 times the direct current voltage Vo is required.
When describing with specific numeric values as an example, in a case of a direct current plasma device with direct current voltage Vo of 800 V, a switching element for reversed-polarity voltage application with rated voltage of 2000 to 2400 V is required. By using an IGBT as a switching element for reversed-polarity voltage application, and serially connecting two units of commercially available low-cost IGBTs with rated voltage of 1,200 V, it is possible to obtain a switching element for reversed-polarity voltage application having breakdown voltage of 2,400 V in total. This switching element for reversed-polarity voltage application having a breakdown voltage of 2,400 V can clearly withstand direct current high voltage before plasma ignition of approximately 1200 to 1500 V corresponding to the voltage of 1.5 to 1.8 Vo as described above.
Although the switching element for reversed-polarity voltage application having such a configuration of serially connecting IGBTs is economically excellent, the switching speed thereof is slower than that of high-speed semiconductor switching elements such as an FET. Therefore, the switching element for reversed-polarity voltage application with a configuration of serially connecting IGBTs has a problem of not being suitable for use as a countermeasure against abnormal electrical discharge for preventing the occurrence of abnormal electrical discharge by applying a high-frequency reversed-polarity voltage pulse to the plasma load. On the other hand, although an FET in general has higher switching speed and smaller ON resistance than an IGBT, many FETs have lower breakdown voltage than IGBTs. For example, although FETs with a rated voltage of 1,200 V are commercially available, an FET with this type of high breakdown voltage has higher ON resistance than a general-purpose FET with breakdown voltage of 500 to 600 V. If two FETs with this type of high breakdown voltage are serially connected, the power loss due to the high ON resistance is increased, and the heat generation is also increased. Moreover, an FET with this type of high breakdown voltage is not suitable for use in generating a reversed-polarity voltage pulse for direct current plasma for reasons such as the switching speed being inferior to a general-purpose FET with breakdown voltage of 500 to 600 V.
In the conventional circuit configuration in which the reversed-polarity voltage is applied to the plasma load as described above, by considering the high surge voltage that occurs when turning off the switching element for reversed-polarity voltage application, there has been a need for a switching element for reversed-polarity voltage application with high rated voltage, and in addition, for a large voltage surge absorption circuit that absorbs such surge voltage. Furthermore, the surge electric power absorbed by the voltage surge absorption circuit is needlessly wasted, and the heat generation of the voltage surge absorption circuit is increased, which is not desirable in terms of economy and environment.